This invention relates to powder coating apparatus, and more particularly to a fluid jet pump for conveying coating powder.
Objects are commonly coated by spraying an electrically charged powder onto the object which is electrically grounded. Electrostatic attraction holds the powder on the object until heat is applied to flow the powder together and to cure it. An apparatus for electrostatic powder coating is typically comprised of an electrostatic voltage generator, a container for holding and suspending powder in fluid, a fluid jet pump for conveying fluidized powder, and a spray gun. Using additional fluid, the jet pump induces a stream of fluidized powder from the container and propels the fluidized powder through a hose leading to the spray gun. The powder particles are electrically charged in the gun and sprayed onto the object. The fluid commonly used is air, although other fluids may be used.
Within the jet pump, a nozzle receives pressurized injector fluid and discharges the injector fluid at high velocity through an injection chamber and into the bore of a jet barrel. The high velocity fluid jet reduces the pressure in the injection chamber thereby inducing a flow of powder particles suspended in fluid from the powder container. The induced suspended powder is propelled by the jet into and through the barrel bore. Typically, the barrel bore discharges the powder stream into a bore through a fitting over which the end of a hose leading to the gun is fitted and fastened.
To control the amount of coating powder sprayed from the gun onto the object, the pressure of the injector fluid is varied to vary the pressure in the injection chamber and thus the rate of induction of fluidized powder from the powder container into the jet pump. As a result, the rate of fluid injected varies. The amount of coating powder desired for spraying may be so reduced, however, that the reduced rate of flow of injector fluid is insufficient to maintain fluidization of the powder within the hose. Consequently, a flow of supplementary fluid is provided which is commonly introduced at the juncture between the jet barrel and the hose fitting.
In operation over time, the bore in the jet barrel, the bore in the hose fitting and junctures between the barrel and the hose fitting are susceptible to buildup and clogging with fused powder particles. This phenomenon is called impact fusion, and is believed to result from the generation of heat in the contact and impact of high velocity powder particles on surfaces and junctures, causing the particles to adhere and cure.
Materials which are slippery and relatively soft alleviate the problem of buildup and clogging by impact fusion. Such a material is polytetrafluoroethylene, as is sold under the trademark, Teflon. Even in such materials, however, junctures exposed to high velocity suspended powder flows are subject to impact fusion.
Surfaces contacted by the high velocity jet of fluid and suspended powder particles in the pump, namely the jet barrel and hose fitting, are subject to erosion and wear and require periodic replacement. Relatively soft materials such as Teflon are particularly subject to erosion and wear by contact with high velocity jets of fluid and suspended powder particles. Thus, more frequent periodic replacement is necessitated of a jet barrel and hose fitting fabricated of material such as Teflon.
Materials such as Teflon are not capable of mass production by injection molding. Useful geometric shapes in such materials must be produced by machining. Machining of a complex object as an integral jet barrel and hose fitting from Teflon is undesirably expensive. Periodic replacement of such a costly object is economically undesirable. Still further, a hose fitting fabricated of slippery material such as Teflon allows a hose to slip off readily. High clamping forces on the hose end to secure retention subject a Teflon fitting of acceptable wall thickness to deformation and collapse.
What is needed is an economical apparatus and method in a jet pump to receive and conduct a high velocity fluid flow of suspended powder from the discharge of a nozzle to the entrance of a hose without clogging by impact fusion. The apparatus desirably should be without juncture in the surfaces contacted by the high velocity flow of suspended powder, capable of convenient periodic replacement in the jet pump, and of simple geometry amenable to inexpensive machining from soft and slippery materials such as Teflon.